Bonsai - Diseases

Similar to people, plants also tend to fall sick less frequently when they are at ease in their environment and are looked after properly. Plants possess a natural resistance which helps to shield them from diseases as well as pests. However, when plants have to compromise too much, for instance live in conditions where air circulation is poor or in extreme hot or cold temperatures or are not looked after and cared for appropriately, they will naturally be susceptible to pests or fungi. In fact, sometimes providing the plants with proper care becomes unavoidable and this may make them fall sick often. Like any other plants, your bonsai trees are also susceptible to diseases when grown in such unfavourable conditions.

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Hence, prevention, which denotes giving due attention and tender care to the plants, is certainly the best way to ensure that your bonsai trees are healthy and will not be susceptible to fungi and pests. Nevertheless, the fact remains that even if you provide the best and most intensive care to your bonsai trees, they can still develop various diseases. In this article, we will be discussing about the most widespread bonsai diseases as well as pests and how to protect your plants from them. It is expected that using this information you will be able to intervene early as well as effectively if and when your plants fall sick. Every one of us who feel affection for nature and plants is really fortunate that compared to the earlier days, currently it is easier to deal with diseases using organic and biological means like using natural products that are also environment friendly. In fact, such organic and biological methods are increasingly replacing chemical substances like synthetic insecticides and fungicides that were earlier used extensively to protect the plants from pests and microbes.

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Powdery mildew

This condition is basically a fungal infection that occurs on the leaf tops. This disease generally occurs owing to poor air ventilation or may even occur when you spray a plant very late in the evening and there is not enough time for the water to evaporate leaving the plants damp and soggy. In fact, genuine powdery mildew can also occur owing to use of any plant fertilizer containing elevated nitrogen concentration. Therefore, it is preferable that you treat your bonsai with a natural or organic anti-mildew fungicide always.

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Downy mildew

If you find that the underside of the leaves of your bonsai has a grey covering and yellowish spots on their surface, you can be sure that the plant has been affected by the condition called downy mildew. This disease is usually attributed to poor air circulation in the region of the bonsai or excessive humidity in the atmosphere coupled with too high water content in the soil. To cure the bonsai of this disease, which is also called false mildew, you should relocate the plant to a new place having a better air circulation. At the same time, you need to spray the relocated affected plant with a good fungicide. The sooner you do this, the better it will be for your bonsai.

You can cure the plants by using a spray prepared by blending one tablespoonful (15 ml) of sodium bicarbonate and two tablespoons of horticultural oil liquefied in 1000 ml (1 quart) of water. This solution can be used for treating both types of mildew - downy mildew and powdery mildew. It has been found that after being sprayed with this solution, some plants turn out to be photosensitive. Nevertheless, you can check with your neighbourhood nursery before spraying this fungicidal solution on your bonsai.


Bonsai trees suffer from chlorosis when their leaves start turning yellow, but the veins continue to be green. This is usually attributed to iron deficiency, which can be avoided or cured by blending any good chelated iron supplement to the water before spraying it on the plants.

Sooty mold

This disease manifests itself in the form of a black substance similar to soot that mainly appears on older leaves. In most cases, sooty mold only appears on a particular side of your bonsai. Usually, such fungus growth occurs after the plant is infested by aphids, also known as plant lice. Aphids secrete a substance called honeydew, which forms the base for such fungal growth. If your bonsai have been affected by sooty mold, treat the plant using a contact insecticide.

Root rot

Your bonsai may develop root rot when water is allowed to stay in the saucer for a prolonged period. Alternatively, root rot may also be caused by excessive feeding of the plants. When a plant develops root rot, its leaves become discoloured or even the branches of the bonsai may snap off. At the same time, the affected roots as well as the root fibers become brownish and soggy. When this occurs, it is important to remove the affected roots immediately. Once you have cut off or removed the affected parts of the plants, you should transplant them using new soil and water it appropriately. During the next one or two weeks, you should reduce watering with a view to ensure that the new roots have the right prospect for growth and can absorb water efficiently. At the same time, it is important to bear in mind that you should abstain from fertilizing the bonsai for a minimum of eight subsequent weeks. In addition, as your bonsai is recuperating from root rot, you should ensure that you do not expose the plant to direct sunlight for some time.


Rust is yet another widespread fungal disease that manifests itself as orange or brownish patches. This disease may also occur in the form of blisters that spread out on the bottom of the leaves. Eventually, the affected leaves shrivel and drop on the ground. These patches as well as blisters are a result of infections caused by the Phragmidium fungus. This particular fungus flourishes in hot and humid climatic conditions. It has been found that providing the plants with fertilizers containing too much potassium often results in rust growth.

Black spot

This is another terrible fungus that generally invades the leaves. Especially, elms are susceptible to black spot. When this fungus infects a plant, its leaves are gradually covered by black patches. Subsequently, the affected leaves curl up and fall from the plant.

Usually, it is easy to treat plants affected by black spot. You can put off or treat black spot by using common fungicides. In case you find that your bonsai has been affected by the black spot fungus, you should use a mancozeb or sulfur based fungicide.

Other deficiency diseases

Deficiency of potassium often leads to several different plant diseases. First of all, deficiency of potassium will destroy the leaves of your bonsai. Initially, the color of the leaves will turn yellow at their edges and gradually wither before dropping on the ground. At the same time, nitrogen shortage may hinder the process of photosynthesis, causing the leaves to become pale and have brownish patches on the plant stalks.

Similarly, deficiency of magnesium, another essential mineral, can discourage flowering, as a result of which the plants gradually produce sparse flowers. Shortage of magnesium may also cause the leaves to become somewhat yellowish.

Deficiency or shortage of phosphorus also hinders flowering, while affecting the leaves. The leaves may retain their green color, but usually fall prematurely owing to the deficiency of this vital mineral.

Fortunately for us, it is possible to cure all the deficiencies mentioned above by providing the bonsai trees with a well balanced feeding program. As a result, it is vital to choose the correct fertilizer for your plants.

While choosing the fertilizer for your bonsai, you need to keep in mind the exact requirements of each individual tree, giving due importance to the tree's growth cycle.


Canker is basically caused by bacterial infections, which manifests in the form of a swelling of the callosities and bark. This disease usually occurs after a plant is pruned using tools that are not sterilized after cutting plants that may be already infected. Therefore, it is advisable that you should pass the blades of your secateurs and grafting knife through flame after pruning/ cutting each individual plant. Canker can be controlled effectively and best by cutting or removing the infected parts of the plants. Cut down an infected branch or get rid of an infected tissue immediately when you notice your bonsai affected by this disease. While cutting the affected branches or tissues, it is advisable that you are careful enough to dress the wounds caused by cutting using mastic, as this will facilitate the healing process as well as prevent the parts from being infected further. At the same time, it is important to burn all the diseased or affected wood soon after removing them from the plant. And never forget to sterilize the tools that were used for cutting them.

Viral diseases

It is fortunate that bonsai growers seldom come across viral diseases. However, if a plant is infected by virus or has been plagued by a viral disease, the only way to cure the same is destroying the affected plant completely with a view to prevent the disease from spreading to new plants and turning into an epidemic of sorts.

Among all viral diseases that affect cultivated plants the most dangerous is definitely mosaic. This viral disease manifests in the form of mottling alternating green and yellow stripes or other different shaped areas on the leaves. When this disease affects a plant, the veins of its leaves are highlighted. Eventually, the leaves wither, become dry and fall on the ground.


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